Semiconductor fabrication methods and apparatus

ABSTRACT

Methods and apparatus for fabricating and cleaning in-process semi-conductor wafers are provided. An in-process wafer is placed within a closed chamber. A reactant gas is incorporated in a liquid solvent to form a “reactant mixture” that is capable of reacting with photoresist material for other material) on a wafer surface to facilitate removal of the material from the wafer surface. The reactant mixture is condensed on one or more of the in-process wafer surfaces to form a thin film on the surface(s) of the wafer. The solvent in the reactant mixture acts as a transport medium to place the reactant gas on the wafer surface. The reactant gas is then able to react with the photoresist material (or other material) on the in-process wafer surface to effect removal the material. Following reaction of the reactant gas with the photoresist, the thin film of reactant mixture is removed from the wafer surface by flash heating, rinsing, draining, or other suitable means.

FILED OF THE INVENTION

The present invention relates to the field of semiconductor devicefabrication and wafer cleaning.

BACKGROUND AND SUMMARY OF THE INVENTION

During the processing of semiconductor wafers used in manufacturingintegrated circuits and the like, it is typically necessary to removechemicals or residues from the wafer surface. For example, it issometimes necessary to etch openings or other geometries into a thinfilm deposited onto (or grown on) the surface of a wafer substrate. (Thewafer substrate typically comprises silicon, gallium arsenide, glass, aninsulating material such as sapphire, or any other substrate materialupon which an integrated circuit wafer may be fabricated.) Presentmethodology for etching such a thin film requires that the film beexposed to a chemical etching agent to remove desired portions of thefilm or films. The composition of the etching agent used to remove theportion of the film depends upon the nature of the thin film.

In order to ensure that only desired portions of the thin film areremoved, a photolithography process is use by which a pattern istransferred to the surface of the thin film. The pattern serves toidentify the areas of the thin film that are to be selectively removed.The pattern is typically formed with a photoresist material, typically alight-sensitive material that is spun onto the in-processintegrated-circuit wafer also in the form of a thin film. The thin filmof photoresist is then exposed to a high intensity light source that isprojected through a photomask. The photomask defines a desired pattern.As the light source is projected through the photomask, the desiredpattern is defined on the photoresist thin film. The exposed orunexposed photoresist, depending upon the polarity of the photoresistmaterial, is dissolved (i.e., is removed or stripped) with developers,leaving a pattern that allows etching to take place in the selectedareas only.

Some of the current methods for stripping the photoresist (or othermaterials, such as dry-etch residues or polymers) include a hot chemicalremoval with a chemical etching agent. Sulfuric acid and hydrogenperoxide or dry reactive removal, known as photoresist ashing aretypical removal methods. The hot chemical removal methods areundesirable in that they involve great expense due to the relativelylarge amount of chemical etching agent needed and require expensivedisposal methods due to the caustic nature of the chemical etchingagents. The ashing method is undesirable in that it involves ahigh-energy gas and often incurs damage to the wafer substrate or thelayers of thin films formed on the wafer substrate to make the waferintegrated circuits.

Some chemicals in the gaseous phase may react with photoresist materialor other such materials to facilitate removal of the materials from thewafer surface. Many of such gases, however, do not have effectivetransport means to the wafer surface to effect the necessary reaction ina reasonable period of time. Also, many such gases are too unstable tobe introduced to the wafer in an atmosphere filled with such gas andeffect the necessary reaction with the photoresist material. Such gasestypically have short half-lives and change in structure in suchenvironments so quickly that the gas is unable to react with a thin filmmaterial on the surface of a wafer. Many such gases (both the unstablegases and those lacking sufficient transport-characteristics), however,are sufficiently soluble in a variety of liquid solvents.

For example, there has been a current interest in the use of ozone(i.e., O₃) as a photoresist etching agent for the stripping ofphotoresist, dry etch residues/polymers, and the like from a wafersurface. Ozone reacts with photoresist material on the wafer surface tooxidize the photoresist (forming CO₂). Ozone, however, is an unstablegas and will decompose before reacting with the wafer surfacephotoresist material if simply introduced in its gaseous state.Accordingly, a solvent is used to dissolve the ozone and transport theozone to the wafer surface such that the ozone may react with thephotoresist material and strip the photoresist material from the wafersurface.

Water may act as a solvent to dissolve ozone. One method for use ofozone as a stripping agent involves immersing the in-process wafer intoa water bath through which ozone is bubbled. It is difficult, however,to get a sufficient amount of ozone dissolved in the water to affect thedesired oxidation reactions. Further, the amount of ozone transported tothe wafer surface is limited due to the large amount of water fillingthe bath. Consequently, the stripping process is very slow.

Without being tied to any particular theory, it is believed that a mainbarrier to dissolution of the gas into water is kinetic in nature.Another method calls for chilling a water bath and using a diffusionplate in the water to bubble ozone gas therethough. The diffusion platecreates numerous tiny bubbles that rise through the water. The wafersare then immersed in the water bath. During this residence time, the gasdissolves in the liquid by crossing the gas/liquid interface so that theozone in the water strips the photoresist (or other material) on thewafer. Other methods for dissolving the gas into the liquid (e.g.,water) include the use of static mixing devices and membrane contactors.

This method, however, relies heavily on the configuration andperformance of the diffusion devices (e.g., the diffusion plate in thewater bath) and requires long time periods of exposure of the gas to thewater. The increased time and the need for diffusion apparatus addundesirable time and complexity to the process. Further, the photoresiststripping effectiveness of such processes is limited, as discussedabove, as only a small amount of ozone moves to (i.e., has physicalcontact with) the surface of the wafer while the wafer is immersed inthe water bath.

Accordingly, methods and apparatus are needed to fabricate and/or cleanwafers without incurring the expense and apparatus complexityencountered with the prior art methods and apparatus. Additionally,methods and apparatus are needed that provide effective stripping ofphotoresist, dry etch residues/polymers, or the like in a reasonablyshort lime period. Further, methods and apparatus are needed that canovercome the kinetic limitation to dissolution of a gas in a liquidwithout the need for long exposure times of the gas to the liquid.

To overcome the disadvantages of the prior art, methods and apparatusare disclosed herein. The methods and apparatus provided eliminate theneed for large amounts of caustic chemical cleaning agents to removephotoresist, dry etch residues/polymers, or the like. The methods andapparatus provided also require only a relatively small amount of gasand liquid to strip the photoresist, dry-etch residues/polymers, or thelike. Additionally, the methods and apparatus overcome the kineticlimitation of dissolution of the gas in the liquid without requiringlong exposure times of the gas to the liquid. Further, the methods andapparatus provide a liquid solvent that effectively transports thereactant gases that most effectively and quickly strip photoresistmaterial (or other material) from a wafer surface. The liquid solvent,however, does not react with materials on the wafer surface, but merelyacts as a transport medium to put the reactant gas in physical contactwith the wafer surface.

More specifically, an in-process wafer is placed in a chamber,preferably a chamber of low volume. A liquid solvent (e.g., water)incorporates (e.g., dissolves) a reactant gas (e.g., ozone) to create a“reactant mixture.” The reactant gas in the mixture will react with andremove photoresist material for other material) on the wafer surface. Inone representative method, the reactant mixture enters the chamber andforms a thin film on one or more surfaces of the wafer. The chamberpreferably includes a reactant gas atmosphere during and/or afterformation of the thin film. The solvent acts as a transport medium toplace the reactant gas in direct physical contact with the wafersurface. The reactant gas is then able to react with the photoresistmaterial (dry etch residue/polymer or the like) on the in-process wafersurface to effect removal of the material. The solvent does not reactwith the photoresist material (or other material at issue) to be removednor with the reactant gas. The solvent acts merely to transport asufficient amount of the reactant gas to the wafer surface such that thegas reacts with the photoresist material to effect removal.

For example, ozone (i.e., a reactant gas for conventional photoresistmaterial) dissolves in water (i.e., an ozone solvent or “transportmedium”) to form a reactant mixture. The reactant mixture condenses toform a thin layer on one or more wafer surfaces. The ozone reacts withthe photoresist to form CO₂. The water does not react with thephotoresist (or the ozone), but merely transports a sufficient amount ofthe ozone gas to the wafer surface so that the ozone gas reacts with thephotoresist to effect removal in a relatively short period of time.Following reaction with the photoresist, the layer of reactant mixtureis removed from the wafer surface by flash heating, rinsing, drained, orother suitable removal method.

In the representative methods and apparatus, the reactant mixturecondenses or otherwise collects on the wafer surface to form a thin filmthereon. The high surface area to volume ratio of the thin film reactantmixture results in transport of a relatively high volume orconcentration of reactant gas directly onto the wafer surface.Accordingly, the removal of photoresist (or the like) from the wafersurface occurs relatively rapidly. The methods and apparatus allow theremoval process to be carried out in a low volume chamber requiring aminimal amount of reactant gas and solvent. Additionally, both thereactant gas and the solvent may be purified and re-circulated duringthe wafer fabrication process, thereby wasting little chemical andhaving little chemical waste for which disposal is necessary.

The foregoing features and advantages of the methods and apparatus willbecome more apparent from the following detailed description ofrepresentative methods and apparatus that proceed with reference to theaccompanying drawings. The present invention is directed toward noveland non-obvious features and advantages of the disclosedmethods/apparatus for fabricating and cleaning in-process wafers, bothindividually and collectively, as set forth above and additionally asset forth in the drawings and description following.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of an in-process wafer.

FIG. 2 is a cross-sectional view of a closed reaction chamber includingin-process wafers therein which wafers are-undergoing one step of aphotoresist stripping method.

FIG. 3 is a general block diagram of a photoresist stripping system.

FIG. 4 is a flow diagram illustrating a number of representativemethods.

DETAILED DESCRIPTION OF THE INVENTION

Methods and apparatus for removing photoresist material, dry etchresidues/polymers, and like materials from an in-process wafer, areprovided. For ease of discussion, the methods and apparatus areprimarily discussed in terms of the stripping of photoresist from anin-process wafer surface. It should be understood that the methods andapparatus may be used to remove a variety of materials and residues fromwafer surfaces and are not limited to the stripping of photoresistmaterial. Further, reactant gases and liquid solvents useful forpracticing the methods are primarily discussed in terms of ozone inwater. It should be understood that a variety of reactant gases(including mixtures of gases) and liquid solvents (as discussed below)may be use to practice the invention.

In general, an in-process wafer is placed within a chamber. A reactantgas is incorporated into a solvent (for example, the liquid solvent maydissolve the reactant gas) to form a “reactant mixture” that is capableof reacting with the photoresist material to facilitate removal of thephotoresist material from the wafer surface.

A number of the representative methods are illustrated in outline formin FIG. 4. In one representative method, the reactant mixture enters thechamber in the vapor phase. A vapor is a gas at a temperature below thecritical temperature. The vapor phase reactant mixture condenses on oneor more of the in-process wafer surfaces to form a thin film or layer onthe surface(s) of the wafer. The solvent acts as a transport medium toplace the reactant gas on the wafer surface(s). The reactant gas is thenable to react with the photoresist material on the in-process wafersurface to effect removal of the material. Following reaction of thereactant gas with the photoresist, the condensed reactant mixture isremoved from the wafer surface by flash heating, rinsing, draining, orother suitable method. The process preferably takes place in a reactantgas atmosphere. Alternatively, preferably the process includes flowing astream of reactant gas over the thin film.

More specifically, a first representative method includes strippingphotoresist material from an upper surface of an in-process wafer, suchas the wafer shown in FIG. 1. A typical, in-process wafer 12 is beingformed on a substrate 14, such as a silicon substrate. A film 16 isdeposited on the substrate 14. A layer of photoresist material 18 isapplied on film 16. The photoresist material 18 is exposed anddeveloped, patterning openings 20. Openings 20 allow subsequent,selective etching of film 16. The photoresist material 18 then needs tobe stripped from the in-process wafer 12.

Accordingly, in a first method, in-process wafers 12 are placed into achamber 10 of an apparatus such as is shown in FIG. 2. The embodiment ofthe reaction chamber 10 shown in FIG. 2 includes in-process wafers 12positioned on the interior 26 of the chamber. The in-process wafers 12are, preferably placed in a boat 24 that supports the wafers in avertical position. Alternatively, a boat or other holder may support thewafers 12 in a horizontal position, or any other position that allowsaccess to the surface of the wafer 12 to be treated.

Chamber 10 is preferably closed to form a controlled environment suchthat its contents are not exposed to the ambient atmosphere. Thus,contaminants, such as oxygen, cannot contact the surface of thein-process wafers 12. Alternatively, chamber 10 may comprise a module(or separate modules) within a tool cluster. A top chamber opening 28communicates on an upstream side of the chamber with a valve 30 and on adownstream side with the interior 26 of the chamber 10. Valve 30 isfluidly connected to a solvent source 34 and to a reactant gas source38. Solvent source 34 may comprise, e.g., a boiler. A bottom chamberopening 40 fluidly communicates with a drain 44 that is connected to are-circulation/purification device, as discussed below. A gas outlet islocated at the bottom of the chamber. A temperature controller, forcontrolling the temperature of the interior 26 of the chamber and/or thewafers 12 in the chamber is connected thereto.

A reactant gas (e.g., ozone) is then dissolved in a liquid solvent(e.g., water). The reactant gas comprises a gas or a mixture of gasescapable of reacting directly with the photoresist material (or othermaterial) on an in-process wafer surface to remove the photoresistmaterial therefrom. Typically, the reactant gases are unstable unlessdissolved in a solvent. The gases also include those gases that arestable but are not transported effectively in sufficient concentrationto a wafer surface because the gas molecules do not remain in physicalcontact with the wafer surface long enough to react with the photoresistmaterial thereon.

The solvents are those liquids that dissolve or otherwise incorporate asuitable concentration of the reactant gas. The solvents also arecapable of forming a film of liquid (or condensate) on a wafer surface.For example, most any perfluorocarbon will dissolve the reactant gasozone. The solvent, however is merely a transport medium for thereactant gas and does not react with the photoresist material (or othermaterial) on the wafer surface. The solvent may comprise a singlesolvent or a mixture of solvents.

In the first method, the reactant gas is first dissolved in the solventto form a “reactant mixture.” The reactant mixture is vaporized (i.e.,volatilized) and introduced to the chamber 10 through upper chamberopening 28. In such a case, the solvent source 34 also includes adissolved gas in the solvent. The concentration of the gas in thesolvent is preferably at least from about 10% to about 95% gas tosolvent by volume. In general, the concentration of the reactant gas inthe solvent should be as high as possible because higher reactant gasconcentrations strip the photoresist material more quickly than doreactant mixtures having lower reactant gas concentrations.

The vaporous reactant mixture enters the interior 26 of the chamber 10to condense on one or both surfaces of the in-process wafer 12 to form afilm or layer thereon. To ensure the vaporous reactant mixture condensesto form a film or layer on the in-process wafer 12, the wafer may becooled to a temperature equal to about the dew point of the solvent. Atsuch a temperature, a thin film of the reactant mixture will form on oneor more wafer surfaces. Alternatively, the wafer 12 may be at ambienttemperature, but with ambient temperature being a temperature that islower than the temperature of the reactant mixture. Good results areachieved when the wafer is at a temperature of about 10° C. lower thanthe temperature of the reactant mixture. Under such conditions, thereactant mixture condenses on one or more of the wafer surfaces. Thewafer preferably is not cooled to a temperature that would freeze thesolvent as freezing may interfere with the transport characteristics ofthe solvent.

In the representative methods, the film or layer of reactant mixtureformed on the in-process wafer 12 surface(s) is preferably from about 1μm to about 100 μm in thickness. At a higher thickness, the reactionprocess slows. A thin film of about 2000 μm to about 3000 μm inthickness tends to cause a relatively, slow reaction process (likely tobe about ten times slower than that of a thin film having a thickness ofabout 100 μm). The reaction time is also slower at lower temperatures,as would be expected.

The reactant gas in the thin film reactant mixture reacts with thephotoresist (or other material) in a relatively short time. Typically,the reactant gas sufficiently reacts with the photoresist material inabout five minutes for a thin film layer having a thickness of about 100μm and a reactant gas concentration of about 10 percent gas by volume.If the thin film is thicker, the reaction time period is longer toensure sufficient removal of the photoresist.

In the first method (as well as the representative methods discussedbelow), the process preferably takes place in an atmosphere of reactantgas. Alternatively, preferably the process includes flowing a reactantgas stream over the film or layer.

Following reaction of the reactant gas with the photoresist (or othermaterial to be removed), the reactant mixture thin film is removed. Thereactant mixture thin film material is removed from the wafer surface(s)by heating the wafer or heating the wafer environment (preferably in hotnitrogen gas). Alternatively, the reactant mixture thin film materialmay be removed by rinsing the wafer 12, allowing the thin film to dripoff the wafer 12, or any other suitable manner (dependent upon thenature of the wafer at issue).

In a second representative method, the reactant mixture is introduced tothe interior 26 of the chamber 10 in the liquid phase by a nebulizer 32in the chamber 10. The nebulizer 32 may comprise an ultrasonic nebulizeror any other suitable device for forming a fine mist of the reactantmixture. The nebulizer 32 creates a fine mist of tiny reactant mixturedroplets. The droplets may be of any suitable size to condense on andform a thin film on the surface of an in-process wafer 12. In oneembodiment, the nebulizer 32 produces reactant mixture droplets at aboutthe Meinhardt droplet size (i.e., about 10 μm to about 50 μm). Anebulizer mist reactant mixture then condenses on one or more of thewafer 12 surfaces. At this point, the second method follows the methodoutlined above in relation to the first method (see FIG. 4).

In a third method, the reactant mixture enters the chamber 10 through ashower device 46 positioned at upper chamber opening 28 of the reactantmixture. The reactant mixture is not in a vapor phase, but “drips” inthe liquid phase from the shower device 46 to the vertically positionedin-process wafers 12. The dripping reactant mixture forms a thin film onthe surface of the wafer 12. With such a method, a thicker layer ofreactant mixture tends to form on the lower portion of the verticallypositioned wafer (due to gravity).

To ensure the desired thin film layer thickness over the entire wafersurface, a hydrophilic material may be put in physical contact with thelower portion of the wafer to remove excess reactant mixture therefrom(thereby ensuring a thin film of suitable thickness remains on the wafersurface). Alternatively, the excess reactant mixture may be allowed tosimply drip off of the wafer surface. A thinner film is desirablehowever, as the thinner film enables a quicker reaction of the reactantgas with the photoresist material, thereby increasing productionthroughput. When using the shower apparatus to form a thin film of thereactant mixture on a wafer surface, the wafer is preferably at atemperature equal to about 25° C. while the reactant mixture ispreferably at a temperature equal to about 90° C. At this point, thethird method follows the method outlined above in relation to the firstmethod (see FIG. 4).

Referring to FIG. 3, in a fourth representative method, a reactant gasfrom gas source 52 is dissolved in a solvent from solvent source 56 toform a reactant mixture. The reactant mixture is then introduced to achamber 62 via valve assembly 30. Valve assembly 30 may include a boiler36 connected to the solvent source 26. Chamber 62 is preferably a closedchamber for the reasons discussed above in relation to the chamber 10apparatus. An in-process wafer 12 is mounted on a spinner device 66. Asthe reactant mixture enters the chamber 62, the mixture is condensed onthe wafer surface 68. The reactant mixture may pass through a mister ornebulizer 42 to form a mist of reactant mixture droplets. A film ofreactant mixture is formed on the surface. The reactant mixture may bespun onto the wafer surface using any conventional spinner device, suchas the spinner shown in FIG. 3 but this is not a preferred method. Thespeed of rotation of the wafer 12 would be used to control the thicknessof the thin film of reactant mixture formed on the wafer surface 68. Atthis point, the fourth method follows the method outlined above inrelation to the first method.

Referring back to FIG. 2, in a fifth representative method, the reactantgas from reactant gas source 38 is introduced to the chamber 10simultaneously with the solvent from the solvent source 34. The solventis introduced to the chamber 10 in the vaporous phase (or liquid phase)such that the reactant gas dissolves in the solvent to form the reactantmixture while in the chamber 10. The reactant mixture then condenses orotherwise forms on the wafer surface, to form a thin film thereon. Therest of the fifth method is carried out as described above in relationto method one (see FIG. 4).

In a sixth representative method, the solvent is vaporized and thencondensed on the wafer 12 to form a film thereon. The liquid solventneed not be vaporized but may form a layer or film on the wafer by anysuitable means (e.g., nebulizing, showering, etc.). The film of solventis then exposed to an atmosphere of the reactant gas within the chamber10. The reactant gas dissolves in the film of solvent and reacts withthe photoresist (or other material) on the wafer. The rest of the sixthmethod is carried out as described above for the first method (see FIG.4).

Lastly, in each of the methods described above, following removal of thereactant mixture thin film (along with removal of any residualphotoresist or other material) the reactant mixture is drained throughlower chamber opening 40 to a drain 44. Drain 44 may be connected to are-circulation/purification device 50 wherein the gas and solvent arepurified and re-circulated to be used for the next batch of wafers.

Whereas the invention has been described with reference to multipleembodiments of the apparatus and representative methods, it will beunderstood that the invention is not limited to those embodiments. Onthe contrary, the invention is intended to encompass all modifications,alternatives, and equivalents as may be included within the spirit andscope of the invention as defined by the appended claims. For example,to form a layer or thin film of solvent or reactant mixture on the wafersurface, the wafer may be dipped into a container of the respectiveliquid.

1-15. (canceled)
 16. A method for semiconductor wafer fabrication, themethod comprising: selecting a liquid solvent that is inert to amaterial on a surface of a wafer; forming a mist of liquid solventdroplets above the surface of the wafer; selecting a reactant gas thatis capable of chemically reacting with the material on the surface ofthe wafer and exposing the reactant gas to the liquid solvent droplets;forming, on the surface of the wafer, a film of the liquid solvent andexposing the film to the reactant gas so that the reactant gas istransported through the film to the material on the surface of thewafer; and cooling the wafer to a temperature equal to or less thanabout a dew point of the liquid solvent.
 17. (canceled)
 18. The methodof claim 16, wherein only one reactant gas is used.
 19. The method ofclaim 16, wherein the film has a thickness of from about 1 micron toabout 100 microns. 20-25. (canceled)
 26. A method of semiconductorfabrication, the method comprising: selecting a liquid solvent that isinert to a material on a surface of a wafer; selecting a reactant gasthat is capable of chemically-reacting with the material on the surfaceof the wafer and incorporating the reactant gas into the liquid solvent;showering the liquid solvent incorporating the reactant gas onto thesurface of the wafer and exposing the liquid solvent to the reactant gasso that the reactant gas chemically reacts with the material on thesurface of the wafer; and controlling the temperature at or near thesurface of the wafer so that the temperature at or near the surface ofthe wafer is less than the temperature of the showering liquid solvent.27. The method according to claim 26, wherein the exposing stepcomprises exposing a film of the liquid solvent to the reactant gaswhile the film is on the wafer surface.
 28. The method of claim 26,wherein the wafer is at a temperature equal to about 25° C. and theliquid solvent is at a temperature equal to about 90° C.
 29. The methodof claim 26, wherein the wafer is supported in a vertical positionrelative to the shower of liquid solvent. 30-31. (canceled)
 32. A methodfor removing photoresist material from a semiconductor wafer, the methodcomprising: selecting a liquid that does not chemically react withphotoresist material; cooling the wafer: forming a layer of the liquidon a surface of the wafer having photoresist material thereon;introducing ozone gas over the layer of liquid such that some of theflowing ozone gas is transported through the layer of liquid to thesurface of the wafer; and reacting the ozone gas transported to thesurface of the wafer with the photoresist material on the wafer surface.33. The method of claim 32, wherein the ozone gas is introduced prior tothe formation of the layer of liquid.
 34. The method of claim 32,wherein the ozone gas is introduced simultaneously with the formation ofthe layer of liquid.
 35. The method of claim 32, wherein the ozone gasis introduced after the formation of the liquid layer.
 36. The method ofclaim 32 in which the liquid layer is less than about 100 microns thickover the majority of the wafer surface containing the liquid layer.37-46. (canceled)